The present invention relates to a multi-layer information recording medium that is typified by an optical disc and that has a plurality of information recording layers.
To increase the surface recording density (recording capacity per medium) of an optical disc, an optical disc apparatus Blu-ray Disc (trademark, hereinafter referred to as BD disc) that uses a blue-violet semiconductor laser (having a wavelength of approximately 405 nm) and a large numerical aperture objective lens has been proposed. In the BD, to decrease the diameter of a light beam spot, a light source having a wave length of 405 nm and an objective lens having a large numerical aperture (NA) of 0.85 are used.
It is assumed that the spot diameter is defined as 1.22×λ/NA. Since a Compact Disc (CD) uses a light source having a wavelength of 780 nm and an objective lens having an NA of 0.45, the spot diameter of the CD is 2.11 μm. Since a Digital Versatile Disc (DVD) uses a light source having a wavelength of 650 nm and an objective lens having an NA of 0.6, the spot diameter of the DVD is 1.32 μm. In contrast, the spot diameter of the BD is as small as 0.58 μm whose spot area is around one fifth of that of the DVD. In addition, since the numerical aperture NA of the objective lens of the BD is increased, an angular error against 90 degrees formed between the disc surface and the optical axis of the laser light beam (this angular error is referred to as the tilt margin) becomes small, the thickness of a cover layer that coats information recording layers can be decreased to as thin as 0.1 mm.
In a one-sided, double-layer disc, an information recording layer formed at a depth of 100 μm from the incident surface of a laser light beam (in a BD-ROM disc, this layer represents a reflection layer, in a recordable BD disc, this layer represents both a reflection layer and a recording layer) is defined as the reference layer (referred to as the 0-th recording layer or the L0 layer) and a recording layer formed at a depth of 75 μm is defined as the first recording layer (or the L1 layer). Thus, to accomplish a larger recording capacity, it is thought that a multi-layer disc having a plurality of information recording layers is hopeful.
N. Shida, T. Higuchi, Y. Hosoda, H. Miyoshi, A. Nakano, and K. Tsuchiya, “The BD-Type Multi-layer 100 GB ROM Disk Using the Photopolymer Sheet,” Technical Digest of International Symposium on Optical Memory, Nara, pp. 10 (2003) (hereinafter referred to as Shida et al.) proposes a multi-payer recording medium as the BD disc.
Shida et al. reports the feasibility of a four-layer BD disc. This document proposes that when materials of intermediate layers (spacers) that compose the four-layer BD disc have different thicknesses, the influence of multiple reflections that occur among the plurality of information recording layers be lightened. FIG. 1 shows the structure of the four-layer BD disc described in the non-patent document 1. The L0 layer is formed on a disc substrate 1 having a thickness of 1.1 mm. The L1, L2, and L3 layers are successively formed on the L0 layer. A light beam transmission layer (also referred to as the cover layer) is formed on the incident side of a laser light beam. The thickness of a spacer interposed between the L0 layer and the L1 layer is 15 μm. The thickness of a spacer interposed between the L1 layer and the L2 layer is 17 μm. The thickness of a spacer interposed between the L2 layer and the L3 layer is 13 μm. Thus, the spacers need to have thicknesses that differ by 2 μm or more.
The spacers of the multi-layer BD disc are often made of a film or a sheet material because a structure of intermediate layers having constant thicknesses can be easily accomplished than the spin coat method using ultraviolet setting resin material. Thus, with this method, a disc medium having six layers or eight or more layers can be accomplished.
However, with respect to a multi-layer medium having four or more information recording layers, in addition to multiple reflections that occur between adjacent spacers, multiple reflections that occur among a plurality of spacers that are not adjacent need to be considered.